i) Field of the Invention
The present invention relates to the use of an austenitic Ni—Cr—Fe alloy as an exterior layer for improved cracking and corrosion resistance of composite tubes used to construct the lower furnace of a kraft (black liquor) recovery boiler.
ii) Description of the Prior Art
A kraft recovery boiler is a chemical reactor which houses a large bed of molten salt. A fuel, consisting of organic residue from the pulping process and spent inorganic cooking chemicals, is fired into the boiler as an aqueous slurry with between 70 and 80% solids content. Corrosion of water wall tubes in kraft recovery boilers represents a significant hazard to the safe operation of the boiler, as contact between the bed of molten salt and water or steam escaping from cracks in the boiler tubes can result in a smelt-water explosion capable of destroying the boiler. One of the most common measures taken to prevent corrosion is the installation of composite tubes to form the boiler walls and floor.
In this context, a composite tube consists of two tubular layers of different materials, one inside the other, joined by a metallurgical bond at the interface between the two. Typically, the inside layer will be made from a carbon steel such as ASTM A-210 or similar. The outer, corrosion-resistant layer of the tube is most often made from UNS S30403 (18-20 Cr, 8-10.5 Ni), bal Fe or, in some cases, variants of either UNS N08825 (38-46 Ni, 19.5-23.5 Cr, 2.5-3.5 Mo) bal Fe or UNS N06625 (20-23 Cr, 0-5 Fe, 8-10 Mo, bal Ni). In addition to the major alloying elements, all of these alloys contain minor amounts of other alloying elements and impurities. The layers can be metallurgically bonded by co-extruding the two materials with a die and mandrel, by welding the outer corrosion resistant layer onto the inner layer, or by depositing, onto the surface of the inner tube, a molten spray of material comprising the composition of the outer layer and allowing it to solidify in place to form the outer layer.
Composite tubes made with UNS S30403 as an outer layer replaced carbon steel tubes in kraft recovery boilers because they possess intrinsically better corrosion resistance, but they have proven susceptible to failure via several different modes, including, but not limited to, general corrosion, thermal fatigue, corrosion fatigue and stress corrosion cracking. Most seriously, cracks have formed in the outer layer of these tubes around primary air port openings, and in locations where they are, or may be, in contact with the smelt bed on the bottom of the boiler. The specific mechanism of cracking has been the subject of many investigations, and a general conclusion has been reached that the mechanism likely varies according to the precise location in the boiler where the cracking occurs.
Replacement tubes made from variants of alloys UNS N08825 and UNS N06625 were introduced into recovery boiler service to alleviate both the cracking and corrosion experienced by composite tubes made with UNS S30403. In particular, the application of UNS N08825 to prevent cracking of boiler bottom tubes was the subject of U.S. Pat. No. 5,324,595. Although improvements in resistance to cracking and corrosion have been noted for each of these alloys in some applications, both have been prone to corrosion and cracking in-service. In fact, none of the alternative materials used thus far represents a universal solution to the various cracking and corrosion found in the lower furnace section of a kraft recovery boiler.
The present invention is based on the discovery of an alloy that offers significantly superior resistance against the combination of cracking and corrosion mechanisms operative in a kraft recovery boiler.